1. Field of the Invention
The invention relates to the transfer of electric energy to a vehicle, in particular to a track bound vehicle such as a light rail vehicle (e.g. a tram) or to a road automobile such as a bus. A corresponding system comprises an electric conductor arrangement for producing an alternating electromagnetic field and for thereby transferring electromagnetic energy to the vehicle. The conductor arrangement comprises a plurality of segments, wherein each segment extends along a different section of the path of travel of the vehicle. At least one of the segments is coupled to a current supply via an associated constant current source adapted to keep the electric current through the segment constant independently of the electric power which is transferred to one or more vehicles traveling along the segment. The invention also relates to a corresponding method of manufacturing the system and to a corresponding method of operating the system.
2. Description of Related Art
Track bound vehicles, such as conventional rail vehicles, mono-rail vehicles, trolley busses and vehicles which are guided on a track by other means, such as other mechanical means, magnetic means, electronic means and/or optical means, require electric energy for propulsion on the track and for operating auxiliary systems, which do not produce traction of the vehicle. Such auxiliary systems are, for example, lighting systems, heating and/or air condition system, the air ventilation and passenger information systems. However, more particularly speaking, the present invention is related to a system for transferring electric energy to a vehicle which is not necessarily (but preferably) a track bound vehicle. A vehicle other than a track bound vehicle is a bus, for example. An application area of the invention is the transfer of energy to vehicles for public transport. Generally speaking, the vehicle may be, for example, a vehicle having an electrically operated propulsion motor. The vehicle may also be a vehicle having a hybrid propulsion system, e.g. a system which can be operated by electric energy or by other energy, such as electrochemically stored energy or fuel (e.g. natural gas, gasoline or petrol).
WO 2010/031593 A1 describes a system and a method for transferring electric energy to a vehicle, wherein the system comprises the features mentioned above. It is disclosed that the system comprises an electric conductor arrangement for producing an alternating electromagnetic field and for thereby transferring the energy to the vehicle. The electric conductor arrangement comprises at least two lines, wherein each line is adapted to carry a different one of phases of an alternating electric current. The conductor arrangement comprises a plurality of segments, wherein each segment extends along a different section of the path of travel of the vehicle. Each segment comprises sections of the at least two lines and each segment can be switched on and off separately of the other segments. Each one of successive segments of the conductor arrangement can be connected via a separate switch for switching on and off the element to a mainline. WO 2010/000495 A1 describes the field of invention and possible embodiments of the conductor arrangement in more detail. In particular, the serpentine-like embodiment of the conductor arrangement can also be chosen for the present invention.
Each segment may be connected to a direct current supply via an inverter for converting the direct current to an alternating current for producing the electromagnetic field. Alternatively, the segments may be connected to an alternating current supply. Combinations are possible, for example two segments may be connected to a direct current supply via a common inverter.
In any case, producing a constant alternating current in the line or lines of the segments has several advantages compared to the operation of the segment at constant voltage. One advantage is that the constant current may be a sinus function of time. This means that only a single frequency of electromagnetic waves is produced. Operating the segment at constant voltage in contrast produces non-sinusoidal functions, which means that harmonics at different frequencies are produced. Furthermore a constant current on the primary side (the side of the conductor arrangement along the track) enables to reduce the size of the receiver for receiving the electromagnetic field on the secondary side (the side of the vehicle).
The constant current source can be realized as a passive network of impedances, which means that none of the components of the constant current source is actively controlled as it would be in the case of a transistor in the line which is used for current limitation.
The constant current source can be located at the input side of an alternating current supply, i.e. a constant alternating current is fed to the segments via the current supply.
However, this does not allow for individual operation of the segments, if the segments are connected in parallel to each other with the current supply. Individual operation of the segments would result in currents through the segments of the varying size.
Therefore, it is preferred for segments, which are connected in parallel to each other with a current supply, to provide a constant current source for each segment which is to be operated individually. Such an individual operation has the advantage that the segment can be switched on while a vehicle is traveling along the segment and can be switched off otherwise.
In order to transfer sufficient power for providing vehicles (in particular trams or busses) for propulsion, currents of at least some tens of Amperes and voltages of at least some tens of Volts are required, i.e. the transferred power should be in the range of at least some kW. In case of a tram, for example, the voltage at the segment may be in the range of 500-1.000 V and the effective current through the segment may be in the range of 150-250 A.
Corresponding impedances, in particular inductances, require heavy components having corresponding large volumes. In addition, these components produce a significant part of the costs of manufacturing such a system.